Apparatus and method for clean, rapidly solidified alloys

ABSTRACT

One non-limiting embodiment of an apparatus for forming an alloy powder or preform includes a melting assembly, an atomizing assembly, and a collector. The melting assembly produces at least one of a stream of a molten alloy and a series of droplets of a molten alloy, and may be substantially free from ceramic in regions contacted by the molten alloy. The atomizing assembly generates electrons and impinges the electrons on molten alloy from the melting assembly, thereby producing molten alloy particles.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application of, and claims priorityunder 35 U.S.C. §121 to, co-pending U.S. patent application Ser. No.12/053,245, filed Mar. 21, 2008, which is a continuation-in-part of, andclaims priority under 35 U.S.C. §120 to, U.S. patent application Ser.No. 11/232,702, filed Sep. 22, 2005.

TECHNICAL FIELD

The present disclosure relates to apparatus and methods for melting andatomizing metals and alloys (collectively referred to herein as“alloys”) under vacuum conditions to produce clean atomized moltenmaterials that can be rapidly solidified as either powders or preforms.The solid preforms may be made from the atomized molten materials usingtechniques such as, for example, spray forming and nucleated casting.Collected powders may be further processed into various articles ofmanufacture. As an example, powders made by such apparatus and methodsmay be collected, containerized, and further processed to consolidatethe powders into solid performs.

BACKGROUND

Current processes used to produce powder metal products typically employconventional fluid atomization techniques to produce alloy powders. Forexample, conventional fluid atomization technology is used to producealloy powders for the production of common pressed and sinteredarticles. Alloy powders also are used in more sophisticated settings,such as in the fabrication of materials from which critical aerospacecomponents are fabricated.

In one conventional fluid atomization process, high pressure gas isimpinged on a molten metal or alloy stream and physically breaks thestream up into small particles of fully or partially molten material. Asthese molten particles dissipate heat, they freeze, and they arecollected as a solid powder. In certain critical applications, such asin the fabrication of certain aerospace components, batches of powderatomized from several small atomization runs are blended, and then theblend is sieved to small size (for example, −325 mesh), containerized ina metallic can, and consolidated into a suitable solid article (preform)by extruding or otherwise compacting the can and its powdered contents.The consolidated article can then be further processed into the desiredshape and character by machining and other conventional techniques.Advantages of this process include the cleanliness, controlled anduniform composition, and relatively small grain size of the consolidatedarticle, which may be critical to the performance of a componentfabricated from the article.

The conventional process, combining steps of melting, atomization,blending, sieving, containerizing, and consolidating, suffers fromseveral drawbacks. For example, the atomized powder from several smallmelts is used to form the blended powder. This is done since a melt mustbe poured through a relatively small orifice during powder formation,and the pour rate is significantly less than is used in casting orconventional melting. Thus, prior to being atomized, the alloy mustremain molten for an extended period, which can result in deteriorationof the alloy's chemical composition, through elemental volatilizationand reactions with the ceramic liner of the melting vessel. Severalsmall melts are atomized so as to minimize compositional deteriorationof any one melt. Accordingly, the powder forming process is typicallytime-consuming and capital intensive. Also, the melts typically areproduced in conventional ceramic-lined furnaces and, hence, theresultant powders are often contaminated with oxides. Once the powdersare formed, they are then handled in several steps, each of whichpresents the possibility, and likelihood, of additional contamination.Also, because the process includes several steps, it is typicallycostly.

Various techniques have been developed to specifically address distinctsteps in the process of forming consolidated articles from a melt usingpowder atomization. Several well known melting techniques have beendeveloped that employ a vacuum environment and do not use aceramic-lined furnace. These techniques result in significantly lessoxide contamination in the melt relative to forming the melt in aconventional ceramic-lined furnace. For example, electron beam (EB)melting technology is now widely known and broadly discussed in thetechnical and patent literature. Another example is the vacuumdouble-electrode remelting (VADER) process, which is known in the artand described in, for example, U.S. Pat. No. 4,261,412. Other knowntechniques of forming molten alloy streams in ceramic-less meltingdevices are disclosed in, for example, U.S. Pat. Nos. 5,325,906 and5,348,566. The '906 patent discloses a melting apparatus combining anelectroslag remelting (ESR) device coupled to a cold induction guide(CIG). In one embodiment described in the '906 patent, a stream ofmolten refined material is produced by melting a consumable electrode inan ESR device. The molten stream passes, protected from the environmentthrough a closely coupled CIG, downstream to a spray forming device. The'566 patent similarly discloses an apparatus combining an ESR deviceclosely coupled to a CIG, but further discloses techniques forcontrolling the flow of molten material through the CIG. The techniquesinclude, for example, controlling the rate of induction heat supplied tothe alloy within the CIG, and controlling the rate of heat removal fromthe molten material within the CIG, through the cold finger apparatusitself and through an adjacent gas cooling means.

In conventional fluid impingement atomization techniques, either a gasor a liquid is impinged on a stream of a molten material. Impingementusing liquid or certain gases introduces contaminants into the atomizedmaterial. Also, given that fluid impingement does not occur in a vacuumenvironment, even impingement techniques using inert gases can introducesignificant impurities into the atomized material. To address this,certain non-fluid impingement atomization techniques that may beconducted in a vacuum environment have been developed. These techniquesinclude atomization processes described in U.S. Pat. No. 6,772,961 B2,entitled “Methods and Apparatus for Spray Forming, Atomization and HeatTransfer” (“the '961 patent”), wherein molten alloy droplets or a moltenalloy stream produced by a melting means coupled with a controlleddispensing means are rapidly electrostatically charged by applying ahigh voltage to the droplets at a high rise rate. The electrostaticforces set up within the charged droplets cause the droplets to break upor atomize into smaller secondary particles. In one technique describedin the '961 patent, primary molten droplets produced by the nozzle of adispensing means are treated by an electric field from a ring-shapedelectrode adjacent to and downstream of the nozzle. Electrostatic forcesdeveloped within the primary droplets exceed the surface tension forcesof the particles and result in formation of smaller secondary particles.Additional ring-shaped field-generating electrodes may be provideddownstream to treat the secondary particles in the same way, producingyet smaller molten particles. The entire disclosure of the '961 patentis hereby incorporated herein by reference.

Electron beam atomization is another non-fluid impingement technique foratomizing molten material, and is conducted in a vacuum. In general, thetechnique involves using an electron beam to inject a charge into aregion of a molten alloy stream and/or a series of molten alloydroplets. Once the region or droplet accumulates sufficient charge theRayleigh limit, the region or droplet becomes unstable and is disruptedinto fine particles (i.e., atomizes). The electron beam atomizationtechnique is described generally in the '961 patent, and is furtherdescribed below.

The '961 patent also discloses techniques using electrostatic and/orelectromagnetic fields to control the acceleration, speed, and/ordirection of molten alloy particles formed by atomization in the processof producing spray formed preforms or powders. As described in the '961patent, such techniques provide substantial downstream control ofatomized material and can reduce overspray and other material wastage,improve quality, enhance the density of solid preforms made by sprayforming techniques, and improve powder quality and yield when atomizingmaterial to a powder form.

In connection with collecting atomized powders, the method of lettingatomized powders settle on the bottom of an atomization chamber is knownand has been routinely used commercially in the manufacture of alloypowders. Also, methods of collecting atomized materials as unitarypreforms, such as, for example, spray forming and nucleated casting, arewell known and have been described in numerous articles and patents.With respect to nucleated casting, specific reference is drawn to U.S.Pat. Nos. 5,381,847, 6,264,717, and 6,496,529 B1. In general, nucleatedcasting involves atomizing a molten alloy stream and then directing theresultant particles into a casting mold having a desired shape. Thedroplets coalesce and solidify as a unitary article in the shape of themold, and the casting may be further processed into a desired component.Spray forming involves directing atomized molten material onto a surfaceof, for example, a platen or a cylinder to form a free-standing preform.Characteristically, the typical solids fraction of the atomizedparticles differs between spray forming and nucleated casting since, forexample, a less fluid and mobile particle is necessary in the mold-lessspray forming process.

As noted above, many of the known processes for melting, atomizing andforming alloys to produce powders and solid preforms have deficiencies.Such deficiencies include, for example, the existence of oxides andother contaminants in the final product, yield losses due to overspray,and inherent size limitations. Accordingly, there is a need for improvedmethods and apparatus for melting and atomizing alloys and formingpowders and solid preforms from the atomized materials.

SUMMARY

One aspect of the present disclosure is directed to a novel apparatusfor forming one of powder and a preform of an alloy. The apparatusincludes a melting assembly, an atomizing assembly, a field generatingassembly, and a collector. The melting assembly is adapted to produce atleast one of a stream and a series of droplets of a molten alloy, andmay be substantially free from ceramic in regions contacted by themolten alloy. The atomizing assembly impinges electrons on molten alloyfrom the melting assembly and thereby atomizes the molten alloy andproduces molten alloy particles. The field generating assembly generatesat least one of an electrostatic field and an electromagnetic field in aregion between the atomizing assembly and the collector. The at leastone field interacts with the molten alloy particles and influences atleast one of the acceleration, speed, and direction of the molten alloyparticles as they pass to the collector. The apparatus optionallyfurther includes a chamber enclosing at least part of the meltingassembly, atomizing assembly, a field generating assembly, and acollector, and a vacuum device provides vacuum to the chamber.

An additional aspect of the present disclosure is directed to anapparatus that may be used to form at least one of a powder and apreform. The apparatus includes a melting assembly providing at leastone of a stream of molten alloy and a series of droplets of moltenalloy, wherein the melting assembly may be substantially free fromceramic in regions contacted by the molten alloy. An atomizing assemblyof the apparatus impinges electrons on molten alloy from the meltingmeans to thereby atomize the molten alloy and produce molten alloyparticles. A field generating assembly of the apparatus produces atleast one of an electromagnetic field and an electrostatic field in aregion of the apparatus downstream of the atomizing assembly. The atleast one field interacts with and influences the molten alloyparticles. In certain non-limiting embodiments of the apparatus, the atleast one field generated by the field generating assembly influences atleast one of the acceleration, speed, and direction of the molten alloyparticles. In addition to a melting assembly, an atomizing assembly, anda field generating assembly, the apparatus optionally further includesat least one of a collector into which the molten alloy particles fromthe atomizing assembly are directed under influence of the at least onefield, and a vacuum chamber enclosing at least part of the meltingassembly, atomizing assembly, and field generating assembly.

Yet another aspect of the present disclosure is directed to a method offorming one of a powder and a solid perform. The method includesproducing at least one of a stream of molten alloy and a series ofdroplets of molten alloy in a melting assembly that is substantiallyfree from ceramic in regions of the melting assembly contacted by themolten alloy. The method further includes generating particles of themolten alloy by impinging electrons on molten alloy from the meltingdevice, thereby atomizing the molten alloy and producing molten alloyparticles. The method also includes producing at least one of anelectrostatic field and an electromagnetic field, wherein the particlesof the molten alloy interact with and are influenced by the field. Themolten alloy particles are collected in or on a collector as one of apowder and a solid preform. In certain non-limiting embodiments of themethod, the particles of molten alloy interact with and are influencedby the at least one field generated by the field generating assemblysuch that at least one of the acceleration, speed, and direction of theparticles of molten alloy is affected in a predetermined manner.

The reader will appreciate the foregoing details, as well as others,upon considering the following detailed description of certainnon-limiting embodiments of apparatus and methods according to thepresent disclosure. The reader also may comprehend such additionaldetails upon carrying out or using the apparatus and methods describedherein.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of apparatus and methods described hereinmay be better understood by reference to the accompanying drawing inwhich:

FIG. 1 is a schematic representation of one embodiment of an apparatusconstructed according to the present disclosure;

FIG. 2 is a schematic representation of aspects of one non-limitingembodiment of an apparatus constructed according to the presentdisclosure, wherein a generally block-shaped field of electrons isgenerated in the pathway of molten material passing through theatomizing assembly;

FIG. 3 is a schematic representation of aspects of one non-limitingembodiment of an apparatus constructed according to the presentdisclosure, wherein a rastering apparatus generates a field of electronsin the pathway of molten material passing through the atomizingassembly;

FIG. 4 is a schematic representation of aspects of one non-limitingembodiment of an apparatus constructed according to the presentdisclosure, wherein a electrons used to produce an electron field in thepathway of molten material passing through the atomizing assembly aregenerated from the outer surface of a filament;

FIG. 5 is a schematic representation of one embodiment of an electronbeam atomizing assembly that may be included in an apparatus constructedaccording to the present disclosure;

FIGS. 6, 7, 7A, 8, and 8A are schematic representations of elements ofdifferent non-limiting embodiments of apparatus constructed according tothe present disclosure, adapted for spray forming a preform;

FIGS. 9 and 9A are schematic representations of alternate non-limitingembodiments of an apparatus constructed according to the presentdisclosure, adapted for forming a powder material;

FIGS. 10-13 schematically illustrate several non-limiting embodiments ofmelting assemblies that may be included in embodiments of apparatusconstructed according to the present disclosure;

FIGS. 14-16 schematically illustrate several non-limiting embodiments oftechniques that may be used to collect solidified atomized materialproduced by embodiments of apparatus constructed according to thepresent disclosure; and

FIGS. 17 and 17A schematically illustrate non-limiting embodiments of anapparatus constructed according to the present disclosure wherein a castarticle is produced in a mold by nucleated casting an atomized moltenalloy produced by electron beam atomization.

DESCRIPTION

In the present description of embodiments and in the claims, other thanin the operating examples or where otherwise indicated, all numbersexpressing quantities or characteristics of ingredients and products,processing conditions, and the like are to be understood as beingmodified in all instances by the term “about”. Accordingly, unlessindicated to the contrary, any numerical parameters set forth in thefollowing description and the attached claims are approximations thatmay vary depending upon the desired properties one seeks to obtain inthe alloys and articles according to the present disclosure. At the veryleast, and not as an attempt to limit the application of the doctrine ofequivalents to the scope of the claims, each numerical parameter shouldat least be construed in light of the number of reported significantdigits and by applying ordinary rounding techniques.

Any patent, publication, or other disclosure material, in whole or inpart, that is said to be incorporated by reference herein isincorporated herein only to the extent that the incorporated materialdoes not conflict with existing definitions, statements, or otherdisclosure material set forth in this disclosure. As such, and to theextent necessary, the disclosure as set forth herein supersedes anyconflicting material incorporated herein by reference. Any material, orportion thereof, that is said to be incorporated by reference herein,but which conflicts with existing definitions, statements, or otherdisclosure material set forth herein is only incorporated to the extentthat no conflict arises between that incorporated material and theexisting disclosure material.

The present invention provides methods and apparatus for enhancing theproduction of powders and solid preforms by processes includingatomization of an alloy. In general, as illustrated in the schematic ofFIG. 1, certain embodiments of an apparatus according to the presentdisclosure, referenced as 100 in FIG. 1, include: a melting assembly(also referred to herein as a “melting device”) 110 that produces atleast one of a stream and a series of droplets of molten alloy; anelectron beam atomizing assembly (also referred to herein as an“atomizing device”) 112 that atomizes molten alloy from the meltingassembly 110 and produces small molten alloy particles; a fieldgenerating assembly (also referred to herein as a “field generatingdevice”) 114 that generates at least one of an electrostatic and anelectromagnetic field and influences at least one of the acceleration,speed, and direction of one or more of the molten alloy particlesproduced by the atomizing assembly 112; and a collector 116 thatreceives molten alloy particles. Also, in general, certain embodimentsof a method according to the present disclosure comprise: producing astream of molten alloy and/or a series of droplets of molten alloy in amelting assembly that is substantially free from ceramic in regions ofthe melting assembly contacted by the molten alloy; generating moltenalloy particles in an atomizing assembly by impinging electrons onmolten alloy from the melting assembly; generating at least one of anelectrostatic field and an electromagnetic field, wherein molten alloyparticles from the atomizing assembly interact with the field, and thefield influences at least one of the acceleration, speed, and directionof the molten alloy particles; and collecting the molten alloy particlesin a collector as a powder and/or as a preform.

As used herein, the terms “melting assembly” and “melting device” referto a source of a stream and/or a series of droplets of a molten alloy,which may be produced from a charge of starting materials, scrap, aningot, or another source of the alloy. The melting assembly or device isin fluid communication with and feeds molten alloy to an atomizingassembly or device. The melting assembly substantially lacks ceramicmaterial in regions of the assembly that are contacted by the moltenmaterial. As used herein, the phrase “substantially lacks ceramic” andthe like means that ceramic either is absent in regions of the meltingassembly that the molten material contacts during operation of theassembly, or is present in regions of the melting assembly that docontact the molten alloy during normal operation but in a way that doesnot result in the inclusion of problematic amounts or sizes of ceramicparticles or inclusions in the molten alloy.

It is important to prevent or substantially limit contact between themolten material and ceramic material in the melting assembly becauseceramic particles can “wash out” of the ceramic linings and mix with themolten alloy. The ceramic particles will have a higher melting pointthan the molten material and may be incorporated into the cast product.Once incorporated into the final product, the ceramic particles canfracture and initiate cracks in the product during low cycle fatigue.Once initiated, cracks can grow and result in product failure. Thus,depending on the intended application for the cast material, there maybe little or essentially no allowance for the presence of ceramicparticles in the material. In conventional cast and wrought metallurgy,ceramic particles from the vacuum induction melting (VIM) step can beessentially removed during the subsequent vacuum arc remelting (VAR)step or, when using conventional triple-melt practice, during theelectroslag remelting (ESR) plus VAR steps. Cleanliness achieved usingvarious practices can be evaluated using a semi-quantitative test knownas the “EB button” test, wherein a sample electrode of the material tobe evaluated is electron beam melted in a crucible and the resultingfloating raft of oxide is measured for the largest oxide present. Inconventional powder metallurgy, the powder is consolidated into productafter melting and there is no means of further refining the product toremove the oxides. Instead, the powder is sieved and the largestfraction of powder that is made into product is that which is equivalentto the smallest defect that the part designers use in their designcriteria. In the design of the most critical aircraft engine parts fromconsolidated powder metals, for example, the smallest modeled defect isapproximately 44 microns and, thus, powders having a sieve size nolarger than this are used. For less critical aircraft engine parts, thesmallest modeled defect could be as large as approximately 149 micronsand, thus, powders having a sieve size no larger than this are used.

Examples of melting techniques that do not introduce ceramic inclusionsthat may be included in an apparatus and used in a method according tothe present disclosure include: melting devices comprising vacuumdouble-electrode remelting devices; melting devices comprising thecombination of either an electroslag remelting device or a vacuum arcremelting device and a cold induction guide; electron beam meltingdevices; and electron beam cold hearth melting devices. However, keepingin mind that an objective of the design of the particular meltingassembly used is to prevent or limit to an acceptable degree any contactbetween the molten material and any ceramics included in the assembly,other melting assemblies that may be used in methods and apparatusaccording to the present disclosure will be apparent to those havingordinary skill.

As used herein, the term “alloy” refers both to pure metals and toalloys. Thus, as non-limiting examples, “alloy” includes, for example,iron, cobalt, nickel, aluminum, titanium, niobium, zirconium, copper,tungsten, molybdenum, tantalum, and alloys of any of these metals,stainless steels, and nickel-base and cobalt-base superalloys.Particular non-limiting examples of nickel-base superalloys that may beprocessed using methods and apparatus according to the presentdisclosure include IN 100 (UNS 13100), Rene 88, Alloy 720, Alloy 718(UNS N07718), and 718Plus™ alloy (UNS N07818) (available from ATIAllvac, Monroe, N.C.). Particular non-limiting examples of titaniumalloys that may be processed using methods and apparatus according tothe present disclosure include Ti-6Al-4V, T-17, Ti-5-5-5-3, and TiAlalloys.

As used herein, the term “atomizing assembly” refers to an apparatusthat impinges at least one stream of electrons (i.e., an electron beam)or a field of electrons on molten alloy from the melting assembly. Asjust used, “impinges” means to bring into contact. In this way, theelectrons imparts a charge to the impinged region of the stream and/orto the individual molten alloy droplets. As discussed in the '961 patentand below, once the charge in a droplet or a particular region of astream reaches a sufficient magnitude, the region or droplet becomesunstable and is disrupted (atomized) into small molten alloy particles.(As used herein, “molten alloy particles” refers to particles thatinclude some content of molten material, but which are not necessarilyentirely molten.) Such an atomizing apparatus may be variously referredto herein as an electron beam atomizing assembly, apparatus, device, orthe like.

Essentially, as discussed in the '961 patent, a fundamental feature ofan electron beam atomizing apparatus is that it is designed to rapidlyapply an electrostatic charge to a stream or droplets of molten alloy.The apparatus is adapted so that the electrostatic charge imparted tothe molten alloy physically disrupts the stream or droplet and producesone or more small molten alloy particles from the molten alloy, therebyatomizing the material. Atomization of molten material using rapidelectrostatic charging through impingement by electrons results in therapid breakup of the material into small particles due to electrostaticrepulsion forces set up within the material. More specifically, theregion or droplet of molten alloy is rapidly electrostatically chargedbeyond the “Rayleigh limit”, such that the electrostatic forces withinthe region or droplet exceed the surface tension of the material and thematerial breaks up into small particles. The Rayleigh limit is themaximum charge a material can sustain before the electrostaticrepulsions set up within the material exceed the surface tension holdingthe material together. Advantages of an atomization technique utilizingthe impingement of electrons on a material to set up electrostaticcharge repulsion with the material include the capability to conduct thetechnique within a vacuum environment. In this way, chemical reactionsbetween the atmosphere or an atomizing fluid with the molten materialcan be limited or eliminated. This capability contrasts withconventional fluid atomization, wherein the material being atomizednecessarily contacts the atomizing gas or liquid and is typicallyconducted in ambient air or in inert gas for titanium-base andnickel-base alloys.

The stream or droplets atomized by the atomizing assembly is produced bythe upstream melting assembly. The melting assembly may include, forexample, a dispenser for forming a suitable stream or droplets. Incertain non-limiting embodiments, such as those disclosed in the '961patent, the dispenser may include a melt chamber having an orifice. Thestream and/or droplets are forced or otherwise emerge from the orificeand pass downstream to the atomizing assembly. In certain non-limitingembodiments, the molten alloy stream or droplets emerge from the orificeof a melt chamber under the influence of mechanical action or pressure.In one possible embodiment, pressure is applied to the molten alloy in adispenser of a melting assembly in a magnitude greater than the pressureon the outside of the dispenser to produce molten alloy droplets at anorifice in the dispenser. Also, in one embodiment the pressure may bevaried so as to selectively interrupt the flow of the molten alloydroplets.

Certain non-limiting embodiments of the melting assembly may be designedto “pre-charge” the molten metal stream or droplets presented to theatomizing assembly with a negative charge. Pre-charging the stream ordroplets would reduce the amount of negative charge required from theelectron beam atomizing assembly to atomize the stream or droplets intosmall particles. One possible technique for pre-charging is to maintainthe melt assembly at a high negative potential relative to otherelements of the apparatus. This can be accomplished by electricallyisolating the melt assembly from other elements of the apparatus, andthen raising the negative potential of the melting assembly to a highlevel using a power supply electrically coupled to the melting assembly.An alternative pre-charging technique is to position an induction ringor plates upstream of the atomizing assembly in a position close to theexit orifice of the melting assembly. The ring or plates, or perhapsother structures, are adapted to induce a negative charge in thedroplets or stream passing a short distance downstream to the atomizingassembly. The atomizing assembly would then impinge electrons on thepre-charged material to further negatively charge and atomize thematerial. Other pre-charging techniques will be apparent uponconsidering the present disclosure.

In certain embodiments of the atomizing assembly according to thepresent disclosure, charge is imparted to the molten alloy stream and/ordroplets by way of a thermionic emission source or a like device. As isknown in the art, the thermionic emission phenomenon, at one time knownas the “Edison effect”, refers to the flow of electrons (referred to as“thermions”) from a metal or metal oxide surface when thermalvibrational energy overcomes the electrostatic forces holding electronsto the surface. The effect increases dramatically with increasingtemperature, but is always present to some degree at temperatures aboveabsolute zero. A thermionic electron gun utilizes the thermionicemission phenomenon to produce a stream of electrons with a well definedkinetic energy. As is known in the art, thermionic electron gunsgenerally comprise (i) a heated electron-producing filament, and (ii) anelectron accelerating region, which is bounded by a cathode and ananode. The filament typically consists of a piece of refractory materialwire, which is heated by passing an electric current through thefilament. Suitable thermionic electron gun filament materials have thefollowing properties: low potential barrier (work function); highmelting point; stability at high temperatures; low vapor pressure; andchemical stability. Certain embodiments of thermionic electron gunsinclude, for example, tungsten, lanthanum hexaboride (LaB₆), or ceriumhexaboride (CeB₆) filaments. Electrons “boil away” from the surface ofthe filament upon application of sufficient thermal energy generated bythe applied current, but electrons produced in this way have very littleenergy. To address this, a positive electrical potential is applied tothe anode. The electrons produced at the filament drift through a smallhole in the cathode, and the electric field in the region between theanode and the positively charged cathode accelerates the electronsacross the gap to the anode, where they pass through a hole in the anodewith a final energy corresponding to the applied voltage between theelectrodes. Thermionic electron guns are commercially available andtheir construction and manner of operation are well known.

In order to negatively charge the droplets or stream to a levelnecessary to overcome surface tension and atomize the material, thedroplets or stream must be subjected to a flow or field of electrons ofsufficient energy and intensity for a finite period of time. Thus, theatomizing assembly preferably produces a “linear” electron field, whichextends a suitable distance along the path traveled through theatomizing assembly by the droplets or stream. A linear electron field,wherein the electrons are spatially distributed, may be contrasted witha point source electron beam emitter, wherein the electrons are focusedin a narrow beam. Spatial distribution of the electrons may be importantin the apparatus of the present disclosure given that the droplets orstream of molten material introduced to the atomizing assembly is movingthrough the assembly under the influence of gravity.

Without intending to be bound to any particular theory, it appears thatelectron beam atomized particles may be formed from a molten droplet orstream by one or both of the following mechanisms. In a first possiblemechanism, new atomized particles are sequentially stripped from thesurface of the droplet or stream as negative charge is added to thedroplet or stream. Another possible mechanism is that atomized particlesare formed by a cascading effect in which the initial molten stream ordroplet breaks up into small particles, the particles are recharged tonegative potential and break up into yet smaller particles, and theprocess repeats during the time in which electrons are added to thesuccessively smaller atomized particles. Under either mechanism, themolten material must be exposed to the electron field for a sufficienttime so that sufficient negative charge accumulates in and disrupts thematerial. One possible spatial distribution of electrons within theelectron field generated in the atomizing assembly is in the form of acylinder of electrons. The longitudinal axis of the cylinder is orientedin the general direction of travel of the molten material through theatomizing assembly. The minimum length of the cylinder (along thelongitudinal axis) required for complete atomization will depend on thetime it takes the free falling molten material to be atomized by theelectron field given the energy and intensity of the electron fieldwithin the cylinder. Non-cylindrical electron field shapes also may beused, such as, for example, fields having a transverse cross-section(transverse to the molten material's general direction of travel throughthe atomizing assembly) that is rectangular, triangular, or some otherpolygonal or otherwise bounded shape. More generally, however, fields ofany combination of energy, intensity, and three-dimensional shapecapable of suitably atomizing the molten material may be used.Non-limiting prophetic embodiments of an electron beam atomizingassembly for an apparatus constructed according to the presentdisclosure are discussed below.

According to one possible non-limiting embodiment of an atomizingassembly according to the present disclosure, a source of electronshaving sufficient energy to atomize the molten droplets or stream isprovided. The electron source may be, for example, a heated tungstenfilament. The electrons stripped from the tungsten filament aremanipulated using electrostatic and/or electromagnetic means to form abeam of electrons having a cross-section that is in the form of arectangle with a large aspect ratio (the ratio of beam width to beamlength). The rectangular-shaped beam is then projected into theatomizing chamber as a generally block-shaped field across the path oftravel of the molten material. FIG. 2 schematically illustrates thisarrangement, wherein atomizing assembly 210 includes tungsten filament212 that is heated by flow of current from power supply 214. Heatedfilament 212 generates free electrons 216. The electrons may begenerated in this way using, for example, a thermionic electron beamemitter. The electrons are shaped by an electrostatic field generated byplates 220 to form a generally rectangular-shaped electron beam 222. Theelectron beam 222 is projected into the interior of the atomizingassembly 210 to produce a generally block-shaped electron field 226.Molten metal droplets 230 dispensed from the upstream melting assembly232 pass through the electron field 226 and are atomized to smallerparticles 238 through disruption by accumulation of negative charge. Theatomized particles 238 pass in the direction of arrow A toward acollector (not shown).

Possible alternative electron generating devices that may be used toproduce electrons in certain embodiments of atomizing assembliesaccording to the present disclosure are cold cathode wire ion generatorsand plasma ion emitters. Cold cathode wire ion emitters typicallyproduce an electron field having a generally rectangular cross-section.One advantage of a cold cathode ion generator is that it produces anelectron emission at temperatures lower than a thermionic electrongenerator. The design of both cold cathode wire ion generators andplasma ion emitters, and their manner of operation to produce electrons,are generally known in the art. Therefore, further description of suchdevices is not provided herein. Electrons produced by the particularelectron-generating device or devices within the atomizing assembly maybe suitably manipulated, such as using electromagnetic and/orelectrostatic fields, to form a beam of electrons having a suitablecross-section. The electron beam may then be projected into theatomizing chamber across the path of travel of the molten material.

FIG. 3 illustrates an additional non-limiting embodiment of an atomizingassembly 310 according to the present disclosure. One or more tungstenfilaments 312 are heated by power supply 314 and produce electrons 316having sufficient energy to atomize molten metal when impinged on themolten metal in sufficient quantities. The electrons may be generated inthis way using, for example, a thermionic electron beam emitter. Theelectrons 316 are manipulated by structures such as, for example, plates320 to form a diffuse spot 322. Rastering apparatus 324 rasters theelectron spot 322 at a high raster rate within the region of theatomizing assembly through which the molten material passes under theinfluence of gravity. The effect of the high raster rate is to provide athree-dimensional electron field 326 having a controlled shape in theatomizing chamber of the atomizing assembly 310 that is large enough tocompletely or substantially completely atomize the molten metal droplets330 introduced by the melting assembly 332 to smaller atomized particles338. The atomized particles 338 pass in the direction of arrow A to acollector (not shown).

A further embodiment of an atomizing assembly useful in an apparatusaccording to the present disclosure is shown in FIG. 4. Atomizingassembly 410 produces an electron field having a large generallyrectangular cross section. The electrons are generated from the surfaceof a generally straight length of tungsten filament 412 heated by powersupply 414. This means of generating electrons contrasts with thetechnique of generating electrons from a point source, as is typicallydone in electron beam guns. The electrons 416 emanating from the surfaceof the filament 412 may be manipulated using electrostatic orelectromagnetic fields, such as, for the example, the electromagneticfield generated by plates 420, to form a beam 422 having a generallyrectangular cross section. The rectangular electron beam 422 may berastered at a high raster rate by a rastering apparatus into theatomizing assembly 410 to form an electron field through which moltenmaterial 430 sourced from melting assembly 432 passes. Alternatively, asshown in FIG. 4, the rectangular electron beam 422 may be projected intothe atomizing assembly 410 by projecting device 424 to form an electronfield 426, having a generally rectangular cross section, through whichmolten material 430 sourced from melting assembly 432 passes. Thematerial 430 is disrupted by accumulation of negative charge intoatomized particles 438, which pass to a collector (not shown) in thedirection of arrow A.

To provide sufficient electrons to suitably atomize molten material, anyof the foregoing embodiments may be modified to include multiple sourcesof electrons at suitable positions within the atomizing assembly.Multiple means for manipulating and projecting/rastering the electronsalso may be utilized to generate a suitable electron field. For examplea number of thermionic or non-thermionic electron beam emitters or otherelectron sources may be oriented at specific angular positions (forexample, three at 120 degrees to one another) about the pathway of themolten material in the atomizing chamber and generate athree-dimensional field of electrons by projecting the electrons fromthe multiple sources into the pathway.

Also, aspects of the several atomizing assembly embodiments describedabove could be combined. For example, in one alternate embodimentcombining aspects of the embodiments shown in FIGS. 2 and 3, therectangular beam 222 of the atomizing assembly 210 is rastered using therastering apparatus 324 in atomizing assembly 310 to produce an electronfield to atomize the molten material. Relative to electron spot 322,rastering the relatively high aspect ratio rectangular electron beam 222may provide larger linear coverage along the path of the molten materialin the atomizing chamber.

In certain embodiments of an electron beam atomizing assembly includedin an apparatus according to the present disclosure, a first flow orstream of electrons is impinged on material emerging from the meltingassembly, thereby atomizing the material to primary molten alloyparticles having a first average size. Impinging a second stream ofelectrons on the primary particles further atomizes the particles to asmaller average particle size. Further reductions in average size may beachieved by impinging additional electron flows or streams on theatomized particles. In this way, several size refinements are possibleusing rapid electrostatic charging by impingement of electrons. Incertain embodiments, rapid electrostatic charging by an electron beam isapplied two, three, or more times along a pathway to achieve a finaldesired average molten material particle size. In this way, the originalsize of molten alloy droplets produced by the melting assembly need notlimit the size of the final atomized particles produced in the atomizingassembly. The multiple electron sources in such an arrangement may be,for example, individual thermionic electron beam emitters, includinglinear thermionic electron beam emitters.

Accordingly, in certain non-limiting embodiments of an atomizingassembly according to the present disclosure, a droplet or a portion ofa stream of molten alloy undergoes two or more stages of atomization tosuccessively reduce the average the size of the resulting atomizedparticles. This may be accomplished, for example, by appropriatelypositioning two or more electron guns or other sources of flows orstreams of electrons along a pathway in a region between the atomizingassembly and the collector. An atomizing assembly having this generalconstruction is schematically illustrated as assembly 500 in FIG. 5. Amelting assembly 512 includes a dispenser 514 that produces a moltenalloy droplet 523 a. The dispenser 514 may use, for example, mechanicalmeans or pressure to produce the molten alloy droplet 523 a from moltenmaterial produced from an ingot, charge, scrap, or other source in themelting assembly 512. Primary electron beam guns 524 a generate streamsof electrons 525 a that impinge on droplet 523 a and impart a negativecharge to the droplet. The electrostatic forces set up within thedroplet 523 a eventually exceed the droplet's surface tension energy,disrupting the droplet and forming primary molten alloy particles 523 b.Secondary electron beam guns 524 b focus streams of electrons 525 b onprimary molten alloy particles 523 b, similarly imparting negativecharge to the particles and disrupting them into smaller secondarymolten alloy particles 523 c. Tertiary electron beam guns 524 c focusstreams of electrons 525 c on secondary molten alloy particles 523 c,also imparting negative charge to the particles and disrupting them intoyet smaller tertiary molten alloy particles 523 d. In one embodiment ofthis arrangement, the several electron beam guns are thermionic electronguns, although any other suitable device for generating suitable streamsof electrons may be used.

As discussed in the '961 patent, “rapid” electrostatic charging refersto charging to a desired magnitude within about 1 to about 500microseconds, preferably about 1 to about 100 microseconds, and morepreferably about 1 to about 50 microseconds. The rapid electrostaticcharging of molten alloy produced by the melting assembly producescharges exceeding the Rayleigh limit of the material, and therebyproduces a plurality of molten alloy particles. The particles, forexample, may have a generally uniform diameter of, for example, about 5to about 2500 microns, more preferably about 5 to about 250 microns.

Accordingly, the atomizing assembly generates molten alloy particles,which are further processed in the apparatus to form either a powder ora monolithic (i.e., one-piece) preform. As used herein, the term“preform” refers to any casting, workpiece, or other article that isformed by collecting together molten alloy particles. In the apparatusand method of the present disclosure, all or a portion of the moltenalloy particles produced by the atomizing assembly are controlleddownstream of the atomizing assembly and collected in a collector. Morespecifically, apparatus according to the present disclosure include atleast one field generating assembly that generates an electrostaticfield and/or an electromagnetic field that is at least partially presentin a region downstream of the atomizing assembly. The electrostaticfield and/or electromagnetic field generated by the field generatingassembly is structured and/or manipulated so as to influence at leastone of the acceleration, speed, direction of the molten alloy particlesthat interact with the field.

As used herein, the term “field generating assembly” refers to anapparatus that generates and, optionally, manipulates, one or moreelectrostatic and/or electromagnetic fields that may be used to controlat least one of the acceleration, speed, direction of molten alloyparticles in a region downstream of the atomizing assembly. Embodimentsof field generating assemblies are described in U.S. Pat. No. 6,722,961B2, which has been incorporated herein by reference.

As used herein, “electrostatic field” can refer to a singleelectrostatic field or a plurality of (two or more) electrostaticfields. An electrostatic field may be generated by, for example,charging a point, plate, or other source to high potential. Also as usedherein, “electromagnetic field” can refer to a single electromagneticfield or a plurality of electromagnetic fields. An electromagnetic fieldmay be created by, for example, passing electric current through aconductor.

In certain embodiments of an apparatus and method according to thepresent disclosure, all or a portion of the molten alloy particlesgenerated by the atomizing assembly and passing within or through thefield(s) produced by the field generating assembly are collected in oron a collector as a powder or a preform. As used herein, the term“collector” refers to an apparatus, element, or portion or region of anapparatus or element, or an assemblage of elements, that is adapted toreceive or collect all or a portion of the molten alloy particlesproduced by the atomizing assembly in the form of a powder or a preform.Non-limiting examples of a collector that may be incorporated intoembodiments of an apparatus or method according to the presentdisclosure include the entirety or a portion or region of a chamber, ahopper, a mold, a platen, a mandrel, or a surface. Typically, thecollector is at ground potential or, preferably, is at a high positivepotential so as to attract the negatively charged atomized particlesgenerated by the atomizing assembly. When the apparatus is adapted tothe formation of a powdered material, such as, for example, a powderedsteel or other alloy, the collector may be, for example, a chamber, ahopper, or some other suitably configured container. When the apparatusis adapted to spray forming an ingot or other preform, the collector maybe, for example, a platen or a mandrel, which may be adapted to rotateor otherwise translate to suitably form a solid article of the desiredgeometry. When the apparatus is adapted for nucleated casting of a solidarticle, the collector typically is in the form of a mold including avoid having the geometry of the desired cast article.

The general arrangement illustrated in FIG. 1, i.e., an apparatuscombining a melting assembly, an atomizing assembly, a field generatingassembly, and a collector, may be designed and operated to produce analloy powder that is retrieved in the collector. In such case, thecollector may be, for example, a chamber, hopper, or other container.The combination also may be adapted to conduct spray forming so as toproduce an ingot or other solid preform on a surface of the collector,which in such case may be, for example, a platen or a mandrel. Thecombination may further be designed to conduct nucleated casting to forma solid cast article on or in the collector, which in such case may be,for example, a mold including one or more side walls.

In certain non-limiting embodiments of an apparatus according to thepresent disclosure designed to conduct spray forming or nucleatedcasting, for example, the directional assembly generates one or moreelectrostatic and/or electromagnetic fields that interact with anddirect molten alloy particles to various regions of the developingpreform at various times during the forming process.

Also, the electrostatic and/or electromagnetic fields can be used todirect molten alloy particles to areas of a developing preform where itis desired to add or remove heat, thereby influencing the macrostructureof the preform. In conducting spray forming or nucleated casting, forexample, the shape of the one or more electrostatic and/orelectromagnetic fields can also be manipulated to produce near-net shapepreforms by directing particles to predetermined regions on thedeveloping preform at various times during the forming or castingprocess. By employing one or more electrostatic and/or electromagneticfields using the field generating assembly, it is possible to enhancethe yield of the forming or casting process, as well as improve (andcontrol) the density of the resulting preform.

Accordingly, the present disclosure describes methods and apparatusincluding means for generating one or more electrostatic and/orelectromagnetic fields for selectively controlling, for example, one ormore of the yield, quality, and density of solid workpieces (preforms)and powders produced from molten material. Methods of directing atomizedmaterials utilizing electrostatic and/or electromagnetic fields in sprayforming and powder atomization are expected to provide significantlyenhanced yields and to provide solid preforms having densities that aresignificantly greater than conventionally-formed preforms.

In one embodiment of an apparatus according to the present disclosure,the field generating assembly generates an electrostatic field in aregion between the atomizing assembly and the collector by electricallycoupling the collector to a high voltage DC power supply and groundingthe atomizing assembly. Given that electron beam atomization is used inthe present apparatus and method and the atomized particles will benegatively charged, negative polarity is used. The electrostatic fieldmay react with the negatively charged molten alloy particles produced bythe atomizing assembly and the particles are influenced to move in thegeneral direction of the electrostatic field lines. This interaction canbe used to control one or more of the acceleration, speed, direction ofthe molten alloy particles toward the collector.

In addition to a high voltage DC power supply, the field generatingassembly included in certain embodiments of an apparatus constructedaccording to the present disclosure can comprise one or more electrodesdisposed at suitable positions and in suitable orientations so as togenerate suitable field(s) between the atomizing assembly and thecollector. The electrodes are positioned and oriented to shape theelectrostatic field between the atomizing means and the collector in adesired manner. The electrostatic field provided under the influence ofthe one or more electrodes can have a shape that directs the moltenalloy particles in a desired manner to the collector.

The field generating assembly can also comprise a plurality of highvoltage DC power supplies, each attached to one or more electrodesdisposed at suitable positions and in suitable orientations between theatomizing assembly and the collector, and that influence the shape ofthe electrostatic field generated by the field generating assemblybetween the atomizing assembly and the collector in a time-dependentmanner. In this way, the field may be manipulated to suitably directmolten alloy particles generated by the atomizing assembly to specificareas or points on the collector or on the developing preform over time.For example, a field generating assembly including a plurality ofelectrodes and associated power supplies can be incorporated in anapparatus according to the present disclosure adapted to producenear-net shape solid articles by spray forming. A field generatingassembly including a plurality of electrodes and associated powersupplies also could be employed to produce solid preforms by sprayforming or nucleated casting having high density relative to preformsproduced by conventional spray forming and nucleated casting apparatus.In such embodiments, the electrostatic field may be varied in terms ofstrength and/or shape to suitably direct the particles of moltenmaterial to the collector in a manner akin to the relatively crudemechanical rastering movement of the atomizing nozzle in a conventionalspray forming or nucleated casting apparatus lacking a field generatingassembly.

In another embodiment of an apparatus according to the presentdisclosure, an electromagnetic field is produced between the atomizingassembly and the collector by one or more magnetic coils positionedintermediate the atomizing assembly and the collector. The magneticcoils are electrically connected to a power supply, which energizes thecoils. Molten alloy particles produced by the atomizing assembly aredirected along the field lines of the electromagnetic field to thecollector. Preferably, the position and/or orientation of the one ormore magnetic coils can be adjusted so as to direct the molten particlesto specific areas or points on the collector or the developing preform.In this way, molten alloy particles can be directed to enhance thedensity of preforms or even produce near-net shape preforms during sprayforming or nucleated casting.

In yet another embodiment of an apparatus according to the presentdisclosure, a plurality of magnetic coils is disposed between theatomizing assembly and the collector. The electromagnetic fieldsgenerated by the plurality of magnetic coils, which may be singly ormultiply energized to different magnetic field intensities, influencethe direction of movement of the molten alloy particles produced by theatomizing assembly, directing the particles to specific predeterminedareas or points on the collector or on the developing preform. By thisarrangement, the molten alloy particles can be directed in predeterminedpatterns to produce, for example, solid preforms having near-net shapeand/or relatively high density. In certain embodiments, the fieldsgenerated by the field generating assembly may be used to improve orrefine the directional control already available through the use oftranslatable atomizing nozzles in conventional spray forming andnucleated casting equipment. In certain embodiments, the substantialdirectional control attainable solely by appropriately manipulatingfield shape, direction, and/or intensity, can entirely replace themovement of atomizing nozzles in conventional spray casting equipment.

Certain embodiments of an apparatus constructed according to the presentdisclosure address the possibility of overspray by suitably charging thecollector. Atomizing a molten stream and/or molten particles using anelectron beam results in particles that are negatively charged due tothe excess of electrons within the atomized particles. By suitablycharging the collector with a charge of opposite sign to the atomizedparticles, the collector will attract the particles and therebysignificantly reduce or eliminate overspray. Overspray is a problematicdrawback of conventional spray forming that can result in significantlycompromised process yields.

Several prophetic embodiments of an apparatus constructed according tothe present disclosure are shown in the following figures and describedin the text below. These prophetic examples are for the purpose ofillustration only, and are not intended to limit the scope of thepresent disclosure or the appended claims. The intended scope of theinvention is better described in the appended claims.

FIG. 6 schematically illustrates certain elements of an embodiment of anapparatus 600 according to the present disclosure that is adapted forspray forming a solid preform. Electron beam atomizing assembly 610produces negatively charged molten alloy particles 612. An electrostaticfield 614 is generated between the atomizing assembly 610 and acollector 616. The atomizing assembly 610 receives at least one of astream and a series of droplets of molten alloy from a melting assembly(not shown) that is substantially free from ceramic in regions thatcontact the molten material. The charged molten alloy particles interactwith the electrostatic field 614, which accelerates the molten alloyparticles 612 toward the collector 616. The molten particles 612 form asolid preform 618 on a surface of the collector 616. The field'sinfluence on speed and/or direction of the molten alloy particles 612may be used to reduce over-spray from the preform 618, thereby enhancingthe yield of the spray forming process, and possibly also increasing thedensity of the preform 618 relative to a density possible without theuse of such a field generating assembly.

FIG. 7 schematically illustrates certain elements of an additionalnon-limiting embodiment 700 of an apparatus constructed according to thepresent disclosure. Melting assembly 710 supplies at least one of astream and a series of droplets of molten alloy to electron beamatomizing assembly 712, which produces a spray of charged molten alloyparticles 714. Electrostatic field 716 is generated by a fieldgenerating assembly between the atomizing assembly 712 and a suitablyshaped collector 718. The field 716 interacts with the charged moltenalloy particles 714 to accelerate the particles 714 toward the collector718. Particles 714 may be accelerated to a greater extent if thecollector 718 is held at a high positive potential. The acceleratingforce and directional control exerted by field 716 on the charged moltenparticles 714 may be used to enhance the density of the solid preform720, and also may be utilized to produce a near-net shape preform 720.The collector 718 may be stationary, or may be adapted to rotate orotherwise suitably translate.

As shown in the alternate embodiment of FIG. 7A, apparatus 700optionally may be modified to include means for generating anon-equilibrium plasma 722 in the path of the molten particles 714between two heat sink electrodes 724. The electrodes 724 thermallycommunicate with an outside thermal mass 726 by way of a dielectricliquid which circulates through conduit 728 under the influence of pumps730. The thermal coupling between the heat sink electrodes 724 and theoutside thermal mass 726 by way of the dielectric fluid allows heat tobe removed from the molten particles 714 and communicated to the thermalmass 726. The non-equilibrium plasma 722 between the heat sinks 724 maybe produced, for example, by means of an AC glow discharge or a coronadischarge. The non-equilibrium plasma 722 transfers heat from the moltenparticles 714 to the two heat sink electrodes 724, which transfer theheat to the outside thermal mass 726. Heat transfer systems or devicesgenerating non-equilibrium plasma and using the plasma to transfer heatto or from atomized molten alloy particles are described in U.S. Pat.No. 6,772,961 B2, the entire disclosure of which has been incorporatedherein by reference. In addition, heat transfer systems and devices forgenerating non-equilibrium plasma and using the plasma to transfer heatto or from articles being cast from molten alloy are described in U.S.patent application Ser. No. 11/008,048, filed Dec. 9, 2004, the entiredisclosure of which is hereby incorporated herein by reference.

FIG. 8 schematically illustrates certain elements of yet anothernon-limiting embodiment 800 of an apparatus constructed according to thepresent disclosure, adapted for spray forming a preform. Meltingassembly 810, which is substantially free from ceramic in regionscontacting the molten material, provides at least one of a flow and aseries of droplets of a molten alloy to an electron beam atomizingassembly 812. The melting assembly 810 optionally may be held at a highnegative potential, such as by optional power supply 822, so as tonegatively “precharge” the molten material before it passes to theatomizing assembly 812, thereby reducing the quantum of negative chargethat the atomizing assembly 812 must convey to the molten material toatomize the material. Such “precharging” feature also may be used withthe other embodiments described herein as a means to, for example,reduce the required quantum of negative charge that must be added to themolten material to atomize the material in the atomizing assembly.Electron beam atomizing assembly 812 produces a spray of charged moltenalloy particles 814. Electromagnetic field 816 is produced by a magneticcoil 818 (shown sectioned). The charged molten alloy particles 814interact with the field 816 and are thereby directed generally toward acollector 820. The directional control of the molten particles 814exerted by field 816 can reduce over-spray, thereby enhancing yield ofthe spray forming process, and also can enhance density of the solidpreform 822.

As shown in the alternate embodiment of FIG. 8A, non-equilibrium plasma842 optionally may be generated in the path of the molten alloyparticles 814 between two heat sink electrodes 844, which are thermallyconnected to an outside thermal mass 846 by a dielectric liquid that iscirculated through conduits 848 by pumps 850. The thermal communicationmaintained between the heat sink electrodes 844 and the outside thermalmass 846 allows heat to be removed from the molten alloy particles 814.The non-equilibrium plasma 842 between the heat sink electrodes 844 isproduced, for example, by means of an AC glow discharge or a coronadischarge. The non-equilibrium plasma 842 also extends from the heatsink electrodes 844 to the electrically grounded solid preform 822 andthe collector 820, providing for heat removal from the preform 822 andthe collector 820. Accordingly, in apparatus 800 heat is transferredfrom the molten alloy particles 814, the solid preform 822, and thecollector 820 by the non-equilibrium plasma 842 to the heat sinkelectrodes 844, and then to the outside thermal mass 846.

FIG. 9 schematically depicts certain elements of an additionalnon-limiting embodiment 900 of an apparatus according to the presentdisclosure, adapted for atomizing molten alloys and forming an alloypowder. Melting assembly 910 provides at least one of a stream and aseries of droplets of a molten alloy to an electron beam atomizingassembly 912. Atomizing assembly 912, which is free from ceramic inregions contacting the molten material, produces charged molten alloyparticles 914. Electromagnetic field 916 produced by a magnetic coil 918(shown sectioned) interacts with the charged molten alloy particles 914to spread out the particles 914 and reduce the probability of theircollision, thereby inhibiting formation of larger molten particles and,consequently, larger powder particles 920. A second electromagneticfield 940 produced by a magnetic coil 943 (shown sectioned) interactswith and directs the cooled particles 942 toward a collector in the formof a hopper 944. The hopper 944 may be remotely sealed by lid 945 andlid closure mechanism 946. The entire powder manufacturing process canbe carried out in a vacuum environment to reduce or eliminatecontamination of the powder 942 by chemical interaction with gases.

Optionally, as shown in FIG. 9A, an alternate embodiment of apparatus900 may be designed so that non-equilibrium plasma 922 is created in thepath of the molten particles 914, between two heat sink electrodes 924that thermally communicate with an outside thermal mass 926 by adielectric fluid which circulates through conduit 928 by force of pumps930. The arrangement of heat sink electrodes 924 thermally communicatingwith outside thermal mass 926 allows heat to be removed from the moltenparticles 914.

As suggested, for example, in connection with the apparatus of FIG. 9,certain embodiment of an apparatus constructed according to the presentdisclosure may include a chamber or the like that encloses or containsall or a portion of the melting assembly, atomizing assembly, fieldgenerating assembly, collector, and workpiece (the powder or preform, asthe case may be). If, for example, a heat transfer device employingnon-equilibrium plasma is incorporated in the apparatus, all or aportion of the heat transfer device and its associated electrodes, aswell as the non-equilibrium plasma, also may be encompassed within thechamber. Such a chamber can be provided to allow for regulating theatmosphere within the chamber, including the species and partialpressures of gases present and/or the overall gas pressure within thechamber. For example, the chamber may be evacuated to provide a vacuum(as used herein, “vacuum” refers to a complete or partial vacuum) and/ormay be completely or partially filled with an inert gas (e.g., argonand/or nitrogen) to limit oxidation of the materials being processedand/or to inhibit other undesired chemical reactions, such asnitridation. In one embodiment of an apparatus incorporating a chamber,the pressure within the chamber is maintained at less than atmosphericpressure, such as from about 0.1 to about 0.0001 torr, or from about0.01 to about 0.001 torr.

Molten alloy particles produced by impinging electrons on moltenmaterial according to the present disclosure generally are highlynegatively charged. Certain embodiments described herein also includemeans to pre-charge molten material with a negative charge, prior toimpinging electrons on and atomizing the molten material. There exists atendency for the negatively charged particles/material to acceleratetoward nearby structures held at ground potential. Such structures mayinclude chamber walls and other apparatus components adjacent the moltenmaterial's path of travel downstream of the melting assembly. In certainnon-limiting embodiments of an apparatus according the presentdisclosure, the atomizing assembly of the apparatus includes plates orother suitably-shaped structures held at negative potential and disposedso as to deflect negatively charged particles/material and inhibitundesirable acceleration of the particles/material toward the chamberwalls and/or other structures held at ground potential.

Accordingly, as included in each of the above prophetic examples,embodiments of an apparatus constructed according to the presentdisclosure include a melting assembly substantially free from ceramic inregions that would contact, and therefore could contaminate, moltenalloy generated by the melting assembly during operation of theapparatus. Each such apparatus also includes an electron beam atomizingassembly to atomize the molten material and generate molten alloyparticles, and a field generating assembly, which generates one or moreelectromagnetic and/or electrostatic fields between the atomizingassembly and a collector and influences at least one of theacceleration, speed, and direction of the particles as they traverse allor a portion of the distance between the atomizing assembly and thecollector.

Optionally, the apparatus further includes means to generate one or morenon-equilibrium plasmas for transferring heat to or from the moltenalloy particles after they are generated by the atomizing assembly, butbefore they are collected as a solid workpiece or as a powder.Alternatively, or in addition, embodiments of an apparatus according tothe present disclosure may generate one or more non-equilibrium plasmasto transfer heat to or from the molten alloy after it is collected on orin the collector, or is applied to a preform developing on or in thecollector.

FIGS. 10-13 schematically illustrate various non-limiting embodiments ofmelting assemblies that may be included as an element of an apparatusconstructed according to the present disclosure. Each such meltingassembly embodiment may be used to produce at least one of a stream anda series of droplets of molten alloy from a consumable electrode orother consumable article. Each such melting assembly embodiment belowmay be constructed so that it lacks ceramic in regions of the embodimentthat would be contacted by the molten alloy generated in theembodiments.

FIG. 10 illustrates use of a vacuum double-electrode remelting device asa component of the melting assembly producing molten alloy that is fedto the electron beam atomizing assembly. The vacuum double-electroderemelting, or “VADER”, technique is well known and is described in, forexample, U.S. Pat. No. 4,261,412. In a VADER apparatus, molten materialis produced by striking an arc in a vacuum between two consumableelectrodes, which melt. An advantage of the VADER technique overconventional vacuum arc remelting (VAR) is that the VADER techniqueallows for much better control of bath temperature and melting rate.Given that VADER apparatus are well known, a detailed description ofVADER apparatus and their manner of operation is unnecessary here.

With reference to FIG. 10, vacuum chamber wall 1010 surrounds theopposed consumable electrodes 1014 and the atomizing assembly 1016.Electric current passes between and through the opposed electrodes 1014,melting the electrodes to produce droplets 1018 (or, alternatively, astream) of molten alloy. The molten alloy droplets 1018 fall from theelectrodes 1014 to the atomizing assembly 1016. The atomized moltenalloy particles produced by the atomizing assembly 1016 pass through andare influenced by one or more electromagnetic and/or electrostaticfields generated by a field generating assembly (not shown), and thenpass onto or into a collector (not shown), examples of which aredescribed below.

FIG. 11 illustrates use of an electron beam melting device as themelting assembly producing molten alloy that is fed to the electron beamatomizing assembly. In electron beam melting, the feedstock is melted byimpinging high-energy electrons on the feedstock. Contamination of themolten product can be avoided by melting in a controlled vacuum. Theenergy efficiency of electron beam melting can exceed that of competingprocesses because of the available control of the electron beam spotdwell time and distribution to the areas to be melted. Also, powerlosses of the electron beam inside the gun and between the gun nozzleand the target material are small. Electron beam melting devices arewell known and, thus, a detailed description of the melting devices andtheir manner of operation is considered unnecessary.

As discussed above, the melting devices described herein, including themelting device of FIG. 11, for example, may be adapted so as to bemaintained at a high negative potential and thereby impart a negativecharge to the molten material before it passes downstream to theatomizing assembly of the apparatus. As an example, the melting deviceshown in FIG. 11 may be adapted to include a melt chamber that iselectrically conductive and maintained at a high negative potential, andwhich the molten material contacts before passing to the atomizingassembly.

Referring to FIG. 11, vacuum chamber 1110 surrounds the melting device'selectron beam sources 1112, the consumable electrode 1114 that is beingmelted, an electron beam atomizing assembly 1116, and a collector (notshown). The electron beams impact the electrode 1114, heating andmelting the electrode to produce droplets 1118 (or, alternatively, astream) of molten alloy. The droplets 1118 fall from the electrode 1114to the atomizing assembly 1116. The atomized molten alloy particlesproduced by the atomizing assembly 1116 pass through and are influencedby one or more electromagnetic and/or electrostatic fields generated bya field generating assembly (not shown), and then pass onto or into acollector (not shown), examples of which are described below.

FIG. 12 illustrates use of an electron beam cold hearth melting deviceas the melting assembly producing molten alloy that is fed to theelectron beam atomizing assembly. In a typical electron beam cold hearthmelting technique, a first electron beam gun melts the charge, which canhave a variety of forms (e.g., ingot, sponge, or scrap). The moltenmaterial flows into a shallow water-cooled crucible (the cold hearth),where one or more electron guns maintain the temperature of the moltenmaterial. A major function of the cold hearth is to separate inclusionslighter or heavier than the liquid material, while at the same timeincreasing the residence time of lower density particles that have ahigh melting point in order to ensure their complete dissolution. All ofthe operations are conducted in a vacuum environment both to ensureproper operation of the electron guns and to avoid alloy contaminationby the ambient environment. An advantage of the electron beam coldhearth melting technique is that it effectively eliminates volatileelements, such as chloride and hydrogen (due to the vacuum), andinclusions in the hearth. The technique also is flexible with respect tothe form of the feed materials. Electron beam cold hearth meltingdevices are well known and, thus, a more detailed description of themelting devices and their manner of operation is considered unnecessary.

Again referring to FIG. 12, vacuum chamber 1210 surrounds the electronbeam sources 1212 and a water-cooled copper cold hearth 1216 of themelting assembly, the consumable electrode 1214 that is being melted, anelectron beam atomizing assembly 1218, and a collector (not shown).Molten material 1220, in the form of a stream and/or a series ofdroplets, falls from the water-cooled copper cold hearth 1216 to theatomizing assembly 1218. The atomized molten alloy particles produced bythe atomizing assembly 1218 pass through and are influenced by one ormore electromagnetic and/or electrostatic fields generated by a fieldgenerating assembly (not shown), and pass onto or into a collector (notshown), examples of which are described below.

FIG. 13 illustrates use of a melting assembly comprising a combinationof an electroslag remelting (ESR) device and a cold induction guide(CIG) to produce molten alloy that is fed to the electron beam atomizingassembly. Alternatively, a melting device combining vacuum arc remelting(VAR) and a CIG may be used in place of the ESR/CIG combination. ESR,VAR, CIG, and melting assemblies comprising ESR/CIG and VAR/CIGcombinations are known. Devices combining ESR or VAR devices and a CIGare known and are described in, for example, U.S. Pat. No. 5,325,906.

In a typical ESR technique, electric current is passed through aconsumable electrode and an electrically conductive slag disposed withina refining vessel and in contact with the electrode. Droplets meltedfrom the electrode pass through and are refined by the conductive slag,and may then be passed to a downstream apparatus. The basic componentsof an ESR apparatus include a power supply, an electrode feed mechanism,a water cooled copper refining vessel, and the slag. The specific slagtype used will depend on the particular material being refined. The VARprocess involves the melting of a consumable electrode composed of thealloy by striking an arc with the electrode in a vacuum. In addition toreducing dissolved nitrogen and hydrogen, the VAR process removes manyoxide inclusions in the arc-plasma. ESR and VAR techniques are wellknown and widely used, and the operating parameters that will benecessary for any particular electrode type and size may readily beascertained by one having ordinary skill in the art. Accordingly,further detailed discussion of the manner of construction or mode ofoperation of ESR and VAR apparatus, or the particular operatingparameters used for a particular material and/or electrode type andsize, is unnecessary.

In the ESR/CIG and VAR/CIG combinations, the CIG, which also isvariously referred to a “cold finger” or “cold wall induction guide”,can maintain the molten material in molten form as the material passesfrom the VAR or ESR apparatus downstream to the atomizing assembly. TheCIG also protects the molten material from contact with the atmosphere.The CIG preferably is directly coupled upstream to the ESR or VARapparatus and downstream to the atomizing assembly so as to betterprotect the refined molten material from the atmosphere, preventingoxides from forming in and contaminating the melt. Certain known designsof a CIG also may be used to control the flow of molten material fromthe ESR or VAR apparatus to the downstream atomizing assembly.

The construction and manner of use of CIG devices is well known and isdescribed in, for example, U.S. Pat. Nos. 5,272,718, 5,310,165,5,348,566, and 5,769,151. A CIG generally includes a melt container forreceiving molten material. The melt container includes a bottom wall inwhich is formed an aperture. A transfer region of the CIG is configuredto include a passage (which may be, for example, generallyfunnel-shaped) constructed to receive molten material from the aperturein the melt container. In one conventional design of a CIG, the wall ofthe funnel-shaped passage is defined by a number of fluid-cooledmetallic segments, and the fluid-cooled segments define an inner contourof the passage that may generally decreases in cross-sectional area froman inlet end to an outlet end of the region. One or more electricallyconductive coils are associated with the wall of the funnel-shapedpassage, and a source of electrical current is in selective electricalconnection with the conductive coils. During the time that the moltenrefined material is flowing from the melt container of the CIG throughthe passage of the CIG, electrical current is passed through theconductive coils at an intensity sufficient to inductively heat themolten material and maintain it in molten form. A portion of the moltenmaterial contacts the cooled wall of the funnel-shaped passage of theCIG and may solidify to form a skull that insulates the remainder of themelt flowing through the CIG from contacting the wall. The cooling ofthe wall and the formation of the skull assures that the melt is notcontaminated by the metals or other constituents from which the innerwalls of the CIG are formed. As is known in the art and disclosed in,for example, U.S. Pat. No. 5,649,992, the thickness of the skull at aregion of the funnel-shaped portion of the CIG may be controlled byappropriately adjusting the temperature of the coolant, the flow rate ofthe coolant, and/or the intensity of the current in the induction coilsto control or entirely shut off the flow of the melt though the CIG; asthe thickness of the skull increases, the flow through the transferregion is correspondingly reduced.

Although CIG apparatus may be provided in various forms, each typicallyincludes: (1) a passage utilizing gravity to guide a melt; (2) coolingmeans in at least a region of the wall to promote skull formation on thewall; and (3) electrically conductive coils associated with at least aportion of the passage, for inductively heating molten material withinthe passage. Persons having ordinary skill in the art may readilyprovide an appropriately designed CIG having any one or all of theforgoing three features for use in an apparatus constructed according tothe present invention without further discussion herein. Given that suchdevices are well known and described in the technical literature, a moredetailed description is considered unnecessary herein.

Again referring to FIG. 13, vacuum chamber 1310 surrounds an ESR/CIGmelting assembly, an electron beam atomizing assembly 1312, and acollector (not shown). The ESR/CIG melt source includes a consumableelectrode 1314 of the desired alloy and a water-cooled copper crucible1316. A heated molten slag 1318 acts to melt the electrode 1314 to forma molten alloy pool 1320. The molten alloy from the molten pool 1320flows through the CIG nozzle 1324, in the form of a molten stream and/ora series of droplets 1322, and passes to the atomizing assembly 1312.The atomized molten alloy particles produced by the atomizing assembly1312 pass through and are influenced by one or more electromagneticand/or electrostatic fields generated by a field generating assembly(not shown), and pass onto or into a collector (not shown), examples ofwhich are described below.

Possible alternative techniques for melting feedstock in the meltingassembly of an apparatus constructed according to the present disclosurewill be apparent to those having ordinary skill in the art. Onenon-limiting example of an alternative melting technique is inductionmelting. In one possible application of induction melting, a coiledprimary electrical conductor may surround a bar of metallic feedmaterial. By passing electrical current through the primary conductor asecondary electric current is generated within the bar throughelectromagnetic induction. The secondary current heats the bar to atemperature greater than its melting temperature.

FIGS. 14-17 illustrate several non-limiting examples of methods that maybe used to collect the solidified atomized material in variousnon-limiting embodiments of apparatus and methods constructed accordingto the present disclosure.

FIG. 14 schematically illustrates an atomized powder being collected inthe bottom of a collector that is a simple chamber. The vacuum chamber1410 encloses an electron beam atomizing assembly 1412. A series ofdroplets of molten alloy 1414 produced by a melting assembly (notshown), which may be, for example, one of the various melting assembliesdiscussed above, passes into the atomizing assembly 1412. The atomizingassembly 1412 produces atomized molten alloy particles 1416, which passthrough, interact with, and are influenced by the electromagnetic and/orelectrostatic field(s) 1413 generated by electromagnetic coil 1417(shown sectioned) of a field generating assembly. The coil 1417 ispositioned to produce the field(s) in the region 1418 downstream of theatomizing assembly 1412. The atomized molten material 1416 is collectedas a powder at the bottom of the chamber 1412.

FIG. 15 schematically illustrates the production of a spray formed solidingot from an atomized molten alloy produced by electron beamatomization using an embodiment of an apparatus constructed according tothe present disclosure. Vacuum chamber 1510 encloses a melting assembly(not shown) and an electron beam atomizing assembly 1512. The meltingassembly may be, for example, one of the various melting assembliesdiscussed above. Droplets of molten alloy 1514 produced by the meltingassembly (not shown) pass into the atomizing assembly 1512. The dropletsof molten alloy 1514 are atomized within the atomizing assembly 1512 toform a spray of atomized molten alloy particles 1516. The atomizedmolten alloy particles 1516 pass through, interact with, and areinfluenced by one or more electromagnetic and/or electrostatic fields(not indicated) generated by plates 1218 of a field generating assembly.The plates 1518 are connected to a power source (not shown) by wires1520 passing through the walls of the chamber 1510. The atomized moltenalloy particles 1516 are directed onto rotating collector plate 1524under influence of the field(s) generated by the field generatingassembly to form a solid preform 1525. The rotating collector plate 1524can be withdrawn downwardly at a rate that maintains the depositioninterface at a substantially constant distance from the atomizingassembly. To enhance yield and improve deposition density, the collectorplate 1524 may be charged to a high positive potential by connecting theplate 1524 to a power supply (not shown) by wires 1526 passing throughthe wall of the chamber 1510.

FIG. 16 schematically illustrates an embodiment of an apparatusaccording to the present disclosure wherein atomized alloy powder iscollected in a can or other suitable container disposed in a firstchamber of the apparatus. The filled container is transferred into asmaller chamber without breaking the vacuum in a vacuum chamber thatencloses some or all of the elements of the apparatus. In the smallerchamber, a lid may welded to the container prior to hot working thecontainer and its powder contents, to produce a consolidated solidarticle. Vacuum chamber 1610 encloses a melting assembly (not shown) andan electron beam atomizing assembly 1612. The melting assembly may be,for example, one of the various melting assemblies discussed above. Aseries of droplets of molten alloy 1614 produced by the melting assembly(not shown) pass into the atomizing assembly 1612. The droplets ofmolten alloy 1614 are atomized within the atomizing assembly 1612 toform molten alloy particles 1616. The molten alloy particles 1616 passthrough, interact with, and are influenced by one or moreelectromagnetic and/or electrostatic fields 1618 generated byelectromagnetic coil 1620 (shown sectioned) of a field generatingassembly. The atomized molten particles 1616 are directed into acollector in the form of a container 1621 under influence of the field1618. When the container 1621 is sufficiently full of powdered atomizedmolten material 1616, it is transferred into chamber 1626, which is thensealed by vacuum lock 1628. A lid can then be secured to the filledcontainer 1621, and the container 1621 may be released to the atmospherevia a second vacuum lock 1630 for thermomechanical processing accordingto known techniques. Optionally, the apparatus of FIG. 16 includes aheat transfer device, such as is generally described above, adapted toremove heat from the molten alloy particles 1616. Also, optionally, thecontainer 1621 is electrically connected to power supply 1624 by wire1622 and is held at a positive potential while the negatively chargedmolten particles 1616 are being collected in the container 1621. Thewire 1622 may be remotely disconnected from the container 1621 beforethe container is moved into chamber 1626.

FIG. 17 schematically illustrates a non-limiting embodiment of anapparatus 1700 constructed according to the present disclosure wherein acast article is produced in a mold by nucleated casting an atomizedmolten alloy produced by electron beam atomization. Vacuum chamber 1710encloses elements including a melting assembly (not shown) and anelectron beam atomizing assembly 1712. The melting assembly may be, forexample, one of the various melting assemblies discussed above. A seriesof droplets of molten alloy 1714 produced by the melting assembly passinto the atomizing assembly 1712. The droplets of molten alloy 1714 areatomized within the atomizing assembly 1712 to form a spray of atomizedmolten alloy particles 1716. The atomized molten alloy particles 1716pass through, interact with, and are influenced by the one or moreelectromagnetic and/or electrostatic fields 1718 generated by theelectrically energized coil 1720 (shown sectioned) of a field generatingassembly. The atomized molten material 1716 is directed into mold 1724under influence of the field 1718 generated by the field generatingassembly, and the resulting solid casting 1730 is withdrawn from themold 1724 by downward movement of the mold base (not shown). Optionally,the mold base may be adapted to rotate or otherwise translate in asuitable manner.

In an alternate non-limiting embodiment of apparatus 1700 shown in FIG.17A, power supplies 1732 are provided and create a potential differenceso as to form a non-equilibrium plasma emanating from the electrodes1734. Heat is conducted by the plasma from the surface of thesolidifying ingot 1730 to the electrodes 1734, which are cooled with adielectric liquid that circulates through heat exchangers 1736 and theelectrodes 1734.

Using various features described above, it would be readily apparent toone of ordinary skill in the art that the foregoing propheticembodiments could be implemented as provided. Moreover, the foregoingembodiments may be modified so as to combine different elementsdescribed herein and provide additional embodiments of apparatus andmethods according to the present disclosure.

Accordingly, certain aspects of the present disclosure are directed toapparatus comprising a melting assembly substantially free from ceramicin regions contacted by molten alloy, an electron beam atomizingassembly, a field generating assembly, and a collector.

Although the foregoing description has necessarily presented only alimited number of embodiments, those of ordinary skill in the relevantart will appreciate that various changes in the apparatus and methodsand other details of the examples that have been described andillustrated herein may be made by those skilled in the art, and all suchmodifications will remain within the principle and scope of the presentdisclosure as expressed herein and in the appended claims. It will alsobe appreciated by those skilled in the art that changes could be made tothe embodiments above without departing from the broad inventive conceptthereof. It is understood, therefore, that this invention is not limitedto the particular embodiments disclosed, but it is intended to covermodifications that are within the principle and scope of the invention,as defined by the claims.

1. An apparatus comprising: a melting assembly configured to produce astream of molten alloy; and an atomizing assembly configured to receivethe stream of molten alloy from the melting assembly, the atomizingassembly configured to generate at least one three-dimensional linearelectron field that impinges on a flow path of the stream of moltenalloy from the melting assembly, wherein electrons impinging on themolten alloy atomize the molten alloy into molten alloy particles. 2.The apparatus of claim 1, wherein the melting assembly is substantiallyfree from ceramic in regions contacted by the molten alloy.
 3. Theapparatus of claim 1, wherein the melting assembly is a ceramic-lessmelting apparatus.
 4. The apparatus of claim 1, wherein the meltingassembly comprises a device selected from the group consisting of avacuum double-electrode remelting device, a device comprising anelectroslag remelting device and a cold induction guide, an electronbeam melting device, and an electron beam cold hearth melting device. 5.The apparatus of claim 1, wherein the atomizing assembly includes anelectron beam emitter configured to emit electrons forming athree-dimensional linear electron field.
 6. The apparatus of claim 1,wherein the atomizing assembly is configured to produce at least one ofan electrostatic field and an electromagnetic field that direct the atleast one three-dimensional linear electron field into the flow path ofthe molten alloy.
 7. The apparatus of claim 1, wherein the atomizingassembly is configured to generate at least one three-dimensional linearelectron field in the form of a cylindrical spatial distribution that isdirected into the flow path of the molten alloy.
 8. The apparatus ofclaim 7, wherein the cylindrical distribution has a longitudinal axisoriented generally in a direction of the flow path of the molten alloy.9. The apparatus of claim 1, wherein the atomizing assembly isconfigured to generate at least one three-dimensional linear electronfield in the form of a rectangular spatial distribution that is directedinto the flow path of the molten alloy.
 10. The apparatus of claim 9,wherein the atomizing assembly is configured to generate and raster arectangular electron beam to provide the rectangular spatialdistribution of electrons.
 11. The apparatus of claim 1, wherein theatomizing assembly is configured to project the electrons to form adiffuse spot and raster the spot to provide a three-dimensional spatialdistribution of electrons having a controlled shape.
 12. The apparatusof claim 1, wherein the atomizing assembly includes an electron beamemitter comprising a filament having a straight length.
 13. Theapparatus of claim 1, wherein the atomizing assembly comprises athermionic electron beam emitter.
 14. The apparatus of claim 1, whereinthe atomizing assembly comprises a plurality of electron beam emitters.15. The apparatus of claim 14, wherein the plurality of electron beamemitters are configured to generate a three-dimensional field ofelectrons impinging on the flow path of the molten alloy.
 16. Theapparatus of claim 1, further comprising a collector selected from thegroup consisting of a surface, a platen, a mandrel, a mold, a chamber,and a can.
 17. The apparatus of claim 16, further comprising a chamberenclosing at least part of the melting assembly, the atomizing assembly,and the collector; and a vacuum device providing vacuum to the chamber.18. The apparatus of claim 16, wherein the collector is held at one of aground potential and a positive potential, thereby attracting negativelycharged molten alloy particles produced by the atomizing assembly. 19.The apparatus of claim 1, wherein the apparatus is configured to form apowder product.
 20. The apparatus of claim 1, wherein the apparatus isconfigured to form a solid preform by a spray forming operation.
 21. Theapparatus of claim 1, wherein the apparatus is configured to form asolid preform by a nucleated casting operation.